The invention disclosed herein relates generally to ultrasonic sensors, and more specifically to single transducer ultrasonic sensors of the type which alternately emit and receive pulses of ultrasonic energy for object detection.
There are at least two general techniques for ultrasonically detecting the presence of an object. One involves transmitting a continuous sinusoidal or sinusoidally modulated wave toward the object, and measuring the phase shift in the wave reflected back therefrom. Transmission of ultrasonic energy and reception of reflections thereof can be accomplished with separate transmitting and receiving transducers. The phase shift which has occurred during signal transit can be determined by phase comparison circuitry. Such a system is disclosed in U.S. Pat. No. 3,577,144 issued to P. Girault on May 4, 1971.
A two transducer continuous transmission system has certain disadvantages. Most obviously, two transducers are required, thus contributing to the cost and physical size of the system. Further, the transducer elements must be closely tuned to one another. The requirement for initially matching the transducer elements contributes to manufacturing cost and complexity. In addition, the transducer elements are subject to frequency drift, thus requiring means for adjusting tuning circuits associated with one or both transducer elements. The tuning circuits and devices further contribute to the cost and complexity of such systems. The tuning process may also contribute to operational time lags, inaccuracies and other limitations.
The foregoing problems and limitations can be avoided in a system using a single transducer for both the transmission and reception of ultrasonic energy. Examples of such systems are shown in U.S. Pat. No. 2,826,753 issued to R. Chapin on Mar. 11, 1958 (reissued Dec. 19, 1961 as U.S. Pat. No. Re. 25,100), and U.S. Pat. No. 3,960,007 issued to E. Swenson on June 1, 1976. In such systems, a standing wave is set up between the transducer and target object. An impedance mismatch, depending on the relative phases of the transmitted and reflected waves, occurs at the transducer and is reflected back into its drive circuitry. This results in variations in the drive voltage which can be used to indicate presence of the object.
Continuous wave systems as previously described are capable of very good detection at relatively short ranges. However, they require relatively high input power since the transmission is continuous, and operational range is relatively short. Attempts to increase range by increasing the input power result in increased heating and rapid deterioration of the transducer element(s).
A second general detection technique is based on reception of a portion of a transmitted pulse reflected from the target object. This can be accomplished either with separate transmitting and receiving transducers or with a single transducer which performs both functions. As in connection with continuous transmission sensors, the transducers in a two transducer pulse transmission sensor must be relatively well matched. The second transducer again contributes to the cost, complexity and size of the sensor package.
Again, at least some of the disadvantages of two transducer pulse transmission sensors are avoided in single transducer implementations. However, single transducer pulse transmission sensors are subject to certain significant limitations because of the effects of mechanical resonance or ringing of the transducer element after each emission of a burst of acoustic energy. The ringing subsides to an acceptable level after a time interval based on damping properties of the transducer. However, until such a level is reached, the transducer is not capable of reliably detecting incoming acoustic energy reflected from a target. This results in inability to detect objects at very short ranges from the transducer. Such a limitation is unacceptable in many ultrasonic sensor applications.
The problem has been dealt with in the fields of ultrasonic flaw detection and medical imaging by using an intermediate member or delay medium between the transducer and the object being tested. U.S. Pat. Nos. 2,467,301 issued to F. Firestone on April 12, 1949, 3,690,154 issued to F. Wells, et al on Sept. 12, 1972 and 3,902,357 issued to R. Soldner, et al on Sept. 2, 1975 disclose blocks of polystyrene or other materials and containers of water or other fluids used for this purpose. In general, the addition of extraneous elements is undesirable because such elements add to the cost and physical size of a system, and may detract from applicability and ease of use. With particular regard to noncontact object detection, the use of intermediate substances as delay media may not be feasible or possible.
Finally, U.S. patent application Ser. No. 550,326 filed Nov. 9, 1983 now U.S. Pat. No. 4,527,360 and assigned to the same assignee as the present application discloses a single transducer pulse transmission ultrasonic distance sensor which provides for range sensing down to zero distance from the sensor housing envelope by mounting the transducer in a housing configured such that an acoustic path at least half as great as the distance travelled by an acoustic signal during the transducer ringing interval is provided within the housing envelope. Provision for such a continuous acoustic path within the housing envelope results in an enlarged envelope which may be undesirable in some applications. In addition circuitry required for sensing distance with such a sensor is more complicated than required or desirable for simply detecting presence of an object.
The applicant has devised a single transducer pulse transmission ultrasonic sensor having a unique housing design and component arrangement without extraneous components which avoids limitations on short range operation so as to permit object detection down to substantially zero distance from the sensor housing envelope.